As a rewritable optical disk, magneto-optical recording disks have been researched and developed, and some of them have had already practical application as an external memory of a computer.
Since a magneto-optical disk, that uses a perpendicular magnetization film as a recording medium, records/reproduces information by utilizing a light, its recording capacity can be enlarged compared to a floppy disk or a hard disc using an in-plane magnetization film. However, the recording density of the magneto-optical disk is limited by a size of a light beam spot on the magneto-optical disk. This is because when a recording bit diameter and intervals of recording bits become smaller than the size of the light beam spot, a plurality of bits enter the light beam spot, thereby making it impossible to reproduce each recording bit separately.
In order to reduce the beam spot so that the recording density is improved, it is effective to shorten a wavelength of the laser light. However, the shortest wavelength of a semiconductor laser on the market is 680 nm, and a semiconductor laser having a shorter wavelength is now being developed. Therefore, it is difficult to further improve the recording density of a magneto-optical disk by using a laser that is now on the market.
On the contrary, for example, "Journal of Magnetic Society of Japan, vol. 19 Supplement No. S1 (1995), pp. 445-446" describes that when a magneto-optical disk having two magnetic layers, namely, a recording layer and a reproductive layer is used for reproduction as a magnetic field is being applied, bits that is smaller than the light beam spot can be read out, thereby making it possible to improve the recording density.
However, in the above method for carrying out reproduction while a magnetic field is being applied in the above manner, since a comparatively strong magnetic field of not less than 800 (Oe) is required for the reproduction, in the case where an electromagnet is used as a magnetic field generating unit, a lot of power consumption is required. Moreover, in the case where a permanent magnet is used as the magnetic field generating unit considering power consumption, there arises a problem that the size of the apparatus is enlarged.